I'm with RD, I'd be walking 20 hrs a day if that was the only exercise I could provide Gunnar. Bruzer isn't so bad, he gets pretty wiped out just keeping up with Gunnar out in the yard. We have a pretty good sized lot, 2/3 acre, and about half of it is fenced.
I do try and walk Gunnar daily, but sometimes it doesn't happen. He definitely gets at least 30 minutes of hard exercise a day out in the yard. I use 2 balls, a former football and a former basketball, throw one, as he returns it, throw the other. I can keep him sprinting for an extended period this way. I might do 10-15 tosses with each ball, then give him a short break, then start again. I also do something where I toss the ball about 5-6' in the air while he's about 20' away, and he'll leap to it and snag it out of the air. As he lands I toss the 2nd ball the same way, he'll drop the 1st and snag the 2nd. I can keep this going for a fair number of tosses before he misses.
The one thing we don't do in the yard is play frisbee, for that we go up to the school, where I can toss it a long distance. 20-30 throws, THEN we go for the walk, lol. I know once I get his panting into double time, he's getting his exercise in.